Kiaran Stapleton, 21, has admitted manslaughter on grounds of diminished responsibility but denies murder.
Bidve's parents are attending the trial at the Manchester Crown Court.
Dr Adrian West, who interviewed Stapleton, told the court today that he asked him why he had selected Bidve from the group of students on Boxing Day. West said, "He replied, 'only because he had the biggest head'".
He quoted Stapleton as saying, "I blew a hole in his head. There is nothing I can say, nothing whatsoever. They should have got a taxi. Things would have been different if they had got a taxi".
When asked about his own situation, Stapleton said, "My life has just turned upside down within the click of a finger."
Asked for his opinion on Stapleton's state of mind as he crossed the road to approach Bidve, West said, "I consider he moved purposefully and then he purposefully asked the time because that gave him an opportunity to have a better select of those people in front of him."
West said he did not think the defendant had a total lack of empathy: "He has impaired empathy. He is still capable of being aware of the feelings and states of other people and the effect of his actions on them."
The court was also told of an incident earlier this year when Stapleton was "purposeful" in throwing a mop bucket of hot water over a fellow inmate in jail.